PROJECT SUMMARY/ABSTRACT Antigen receptor signaling to NF-?B is a highly regulated, critical pathway for B and T lymphocyte activation during the adaptive immune response. NF-?B controls many genes required for lymphocyte function including genes that promote proliferation and survival. The inappropriate activation of NF-?B is associated with multiple lymphomas, which frequently acquire mutations in signaling molecules that elicit their receptor-independent constitutive NF-?B activity. CARD11 is a key scaffold protein that functions in both T cell receptor and B cell receptor pathways to transmit signals from the engaged receptor to the activation of the IKK complex and NF- ?B. Aberrant CARD11-dependent signaling is required for the dysregulated proliferation of the activated B cell- like (ABC) subtype of Diffuse Large B Cell Lymphoma (DLBCL), and mutations in CARD11, which hyperactivate the protein, are found in ~10% of patient samples of ABC DLBCL. Previous work has established that during normal signaling, CARD11 undergoes a transition from an inactive to an active signaling scaffold that recruits several signaling cofactors into a complex that induces IKK activity. An Inhibitory Domain (ID) in CARD11 controls this transition; it keeps CARD11 inactive in the basal state, but receives signals from the engaged receptor that neutralize its inhibitory action and allow CARD11 to signal. Lymphoma-associated mutations in CARD11 bypass normal activation and convert CARD11 into a constitutively active signaling scaffold. However, it remains poorly understood how during normal antigen receptor signaling CARD11 is converted to its active state, how precisely normal and oncogenic forms of CARD11 signal to the IKK complex to activate NF-?B, and how DLBCL-associated gain-of-function CARD11 alleles achieve the transformation of normal B cells into lymphoma. In this application we will 1) investigate how signaling potential and cooperating mutations determine the extent of oncogenic CARD11-mediated aberrant B cell proliferation in vivo; 2) investigate the role of a newly identified factor required for CARD11 activity in normal and oncogenic antigen receptor signaling; and 3) dissect determinants required for several steps in the CARD11 signaling cycle. Our studies will advance understanding of the mechanisms of antigen receptor signaling during lymphocyte activation, illuminate how signaling proteins, especially scaffolds, achieve signal-induced activation during normal physiological behavior, and improve our understanding of how the combination of genetic lesions found in lymphoma determines disease severity. Our studies should reveal a previously unrecognized molecular target for the development of new therapies designed to treat NF-?B-dependent cancers and other diseases that result from aberrant immune cell behavior.